Abstract: Methods, systems, and apparatus, including computer programs encoded on computer storage media, for presenting content items to users. One of the methods includes within a user interface that comprises at least one of: a single window or a single tab of a web browser or an augmented reality environment or a virtual reality environment, displaying one or more graphical units each containing one or more items, each of the items comprising a content item or a control, the items of each of the graphical units being associated with a particular application, at least two of the graphical units being displayed simultaneously at times, the at least two graphical units being displayed in respective positions in a horizontal array in the user interface, and in response to a user invoking one of the content items or controls of one of the graphical units, altering a graphical display characteristic of another one of the graphical units.